Bugfix/main css in main chunk

[KaneFreeman]

Type: bug

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier as per the readme code style guidelines
• Unit or Functional tests are included in the PR

Description:
Bootstrap plugin will now import the main.css file when the path is provided from cli-build-app (aka main.css exists).

Resolves dojo/cli-build-app#228

[codecov]

Codecov Report

Merging #335 (8801981) into master (181c774) will decrease coverage by 0.02%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #335      +/-   ##
==========================================
- Coverage   85.43%   85.41%   -0.03%     
==========================================
  Files          54       54              
  Lines        2602     2605       +3     
  Branches      706      707       +1     
==========================================
+ Hits         2223     2225       +2     
- Misses        379      380       +1     

Impacted Files	Coverage Δ
src/bootstrap-plugin/async.js	0.00% <0.00%> (ø)
src/bootstrap-plugin/BootstrapPlugin.ts	100.00% <100.00%> (ø)

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